I opened up system prefs to calibrate my screen for the first time so you will need to go to 'Displays' then 'Color' then 'Calibrate' on the side.
It will open up a few tests and when finished make a new display profile, when your done constantly switch between your original profile and the calibrated one (imac calibrated).
You will see a huge difference in enhancement of the screen color lighting and adds more texture, you should look at your wallpaper when switching I chose the snowy mountain wallpaper from snow leopard and it really improved a lot.
It will open up a few tests and when finished make a new display profile, when your done constantly switch between your original profile and the calibrated one (imac calibrated).
You will see a huge difference in enhancement of the screen color lighting and adds more texture, you should look at your wallpaper when switching I chose the snowy mountain wallpaper from snow leopard and it really improved a lot.